1. Field of the Invention
This invention relates to the field of loudspeakers, and especially to high powered loudspeakers, including public address systems and acoustical warning devices.
2. Description of the Prior Art
Use of a horn-like member having expanding cross-sectional area moving away from an acoustic source is well known and applicable to many environments. The general idea of such a horn is to direct the acoustical energy along the axis of the horn. Sound waves produced at the source will move outward through the horn, and upon reaching the point at which the cross-section of the horn corresponds to the wavelength at that frequency, the sound wave is directed outwards by the horn. In the range of audible frequencies, wavelengths vary greatly. For example, a minimum audible frequency of 20 Hz in atmospheric pressure (speed of sound 342 Meters second) results in a wavelength of 17.1 meters. A frequency of 20 KHz results in a wavelength of 1.7 cm. In order to accomodate the wide range of frequencies as well as dimensional limitations for practical transportation, many different speakers and horns have been devised.
Such horns, originally used to direct the energy of voices or musical instruments, evolved naturally into the electrically driven loudspeakers of today. Requirements of high efficiency and uniform projection of acoustical energy over a wide range induced those skilled in the art to develop many different theories and dimensional preferences regarding speaker horns. With regard to the efficiency of electrically driven speakers, there are two considerations to be taken into account. A first consideration involves the electrical efficiency in conversion of electrical power into acoustical power. A less obvious consideration impacting on efficiency is to confine the projected acoustical power to the field in which the listeners are located. In other words, a loudspeaker which directs power uniformly in all directions is only "efficient" if listeners are located in all directions. In the usual case, listeners are located only in a restricted field or audible range, and although environments of speakers vary widely, it is usually true that acoustical power projected widely in a vertical direction is wasted.
In the particular environment of acoustical alarm devices, such as firehouse sirens, air raid warnings and the like, wherein the warning must project several miles, wasting projected power in vertical directions is particularly unacceptable. In such devices, the optimum system would project a beam of acoustical energy horizontally, and the loudspeaker would be mounted at some position above the ground, whereby a range of persons in a horizontal window would be subjected to the alarm. Listeners concerned with the fidelity of sound reproduction at concerts and the like require not only directional control but good frequency response over the entire audible range. Accordingly, there is a need for a loudspeaker which can not only confine the projected acoustical energy to a certain plane or beam but can do so with high fidelity.
Persons skilled in the art of loudspeakers have expended substantial energy in developing directional loudspeakers in which a range of frequencies would be uniformly projected at equal power without regard to the listeners' positions within the field. This could, of course, be accomplished by use of a large number of speakers located at various positions close to each portion of the listening audience. Multicellular speakers having a large number of loudspeaker horns directed at a plurality of angles have been designed. Conversely, single loudspeaker horns have been designed which attempt to limit dispersion in certain directions. In U.S. Pat. No. 2,690,231--Levy et al, a speakerhorn is disclosed wherein a planar vertical flare close to the sound source interfaces with substantially outwardly-directed top and bottom faces approaching the aperture. In horizontal cross-section, Levy et al employs an exponentially flared horn. The Levy design is asserted to limit dispersion in a vertical or upward direction using a point source of sound energy.
U.S. Pat. No. 4,071,112--Keele, Jr. also purports to limit dispersion in the vertical direction. Similar to the design of Levy et al, Keele teaches a speaker which flares differently in the vertical and horizontal directions. Although the Keele loudspeaker expands both horizontally and vertically, the horizontal expansion is much more substantial. Finally, U.S. Pat. No. 4,187,926--Henricksen et al teaches a loudspeaker having a rectangular cross-section near the point source, longer in the vertical dimension, expanding into a substantially square aperture. The differing flares in the side walls of the loudspeaker horn are asserted to be useful to control the directivity of the speaker, and a number of options are taught whereby the designer can maximize the efficiency of sound projection at various frequencies from a point source.
The present invention has foresaken the prior art's insistence on a source of sound, and quite unexpectedly, limits vertical dispersion by utilizing a multiple driven line source which extends in the very directions in which dispersion must be limited. Although, the dispersion is still dependent to some extent on the frequency of the signal and the geometry of the speaker horn, this invention provides uniform transmission characteristics over its design horizontal coverage, independent of the frequency.